1. Introduction
This invention relates generally to electromagnetic flowmeters, and in particular to an electromagnetic flow measuring tube comprising an inner member formed substantially of plastic and an outer member which surrounds the inner member.
2. Description of the Prior Art
In an electromagnetic flowmeter, a magnetic field is generated in a flow measuring tube through which the fluid to be metered is conducted. The lines of flux of the magnetic field are perpendicular to the longitudinal axis of the flow measuring tube and to a transverse axis along which two electrodes are located at diametrically opposed positions. According to Faraday's law of induction, a voltage proportional to the flow velocity is induced in the fluid flowing through the flow measuring tube if the fluid is electrically conducting. This voltage is measured between the two electrodes and depends inter alia on the electrode spacing and the strength of the magnetic field. These two quantities are influenced by the geometrical dimensions of the flow measuring tube so that it is desired that the mechanical stability of the flow measuring tube be as high as possible.
As disclosed in U.S. Pat. No. 4,186,600, it is known for this purpose to provide a rigid tubular outer member, preferably a metal tube, the interior surface of which is coated with a plastic liner which is applied in a hot condition, usually by injection molding. The plastic liner serves to insulate the outer member from the electrically conducting fluid and also to protect it from damage if the fluid is chemically aggressive or corrosive. Compared with metals, plastic generally has a high thermal coefficient of expansion and low mechanical strength, in particular as regards tension, pressure and shearing. In the prior art flow measuring tubes of this kind, the plastic liner does not contribute to the mechanical stability of the flow measuring tube which is entirely provided by the rigid tubular outer member. If the flow meter is provided with mounting flanges, these are parts of the rigid tubular outer member.
When the plastic liner cools down after completion of the injection molding, the high thermal coefficient of expansion leads to a shrinkage of the plastic liner. This shrinkage can cause an at least partial detachment of the inner lining from the outer member and permanent mechanical stresses in the plastic. The detachments change the geometrical dimensions of the cross-section of the flow measuring tube, and this can lead to falsifications of the measuring voltage causing measuring errors. The mechanical stresses which are undissipated increase the risk of cracks occurring in the plastic.
It is known from U.S. Pat. No. 4,592,886 to fill the gaps formed by the detachments between the metal outer member and the plastic inner liner with a suitable material, for example a two-component epoxy, to insure in this manner the dimensional stability of the measuring tube cross-section. Introducing the filler into the intermediate spaces is however difficult. In addition, formation of a homogeneous internal cross-section is complicated.